In many computer applications, it is desirable to protect the contests of the computer memory from unlawful or unauthorized intrusion with an intent to extract and read its contents. It is conventional practice to prevent reading of information electronically by providing certain encryption schemes wherein data is transmitted and received in an encrypted form and only authorized people who have the decryption key are able to read the data. There are many different types of encryption schemes which are useful in protecting the sensitive data against being read by unauthorized persons. Encryption keys and other sensitive data are often stored in I/C (integrated circuit) memory components within the computer. By use of software, the stored information is generally adquately protected from unauthorized persons using keyboard entries to attempt memory interrogation. However, an unauthorized person with the necessary skills and knowledge, and sufficient motivation can bypass software controls and attack the computer hardware directly. There are many attacks, some straightforward and well known, others more sophisticated, that allow direct interrogation of memory components and devices. One scheme of protection against such attacks is to provide some type of detecting means which detect any attempted mechanical intrusion into the sensitive area of the computer and, when such intrusion is detected, an alarm is given and/or a signal is sent to circuitry, which circuitry erases the data, thereby preventing the compromise of the information which was stored in the computer memory components. Various schemes have been proposed which provide for some type of electronic or electrical grid surrounding the computer circuitry and, when this electrical grid is broken or breached, the requisite signal is generated. Early schemes for such electronic detection are shown in U.S. Pat. Nos. 4,446,475 and 3,594,770. However, these early schemes have several drawbacks. One such drawback is that many grids are susceptible to very careful mechanical manipulation to allow the memory device to be accessed without breaking or otherwise compromising the circuit. In addition, some of these systems are susceptible to a type of attack wherein the materials which support the electrical grid are chemically attacked leaving access areas exposed to circumvent the electrical grid thus allowing physical intrusion into the memory components.
Still other more sophisticated attacks, through temperature modification or through ionizing radiation (e.g. x-rays) affect volatile memory devices such that an erasure command is not effective, thereby allowing the electrical grid to be circumvented.
A better scheme of protecting computer memory, is given in U.S. Pat. Nos. 5,027,397 and 5,159,629. This system overcomes the limitations mentioned previously by providing an outer intrusion detection layer that is highly resistant to chemical and mechanical attacks. The barrier includes a screen material surrounding the electronic assembly. The screened material has formed thereon fine conductive lines in close proximity to each other in a pattern that limits the mechanical access which can be achieved without disturbing the resistive characteristics of at least one line or line segment. The lines are formed using conductive particles of material dispersed in a solid matrix of material which loses its mechanical integrity when removed from the screen substrate. Electrical supply and signal detection means are provided which are adapted to supply a signal to the conductive lines and generate an output signal responsive to a given change in the resistance of the conductive lines whereby, when the resistance of the conductive lines changes, either as a result of chemical attack or mechanical attack, a signal is generated. This signal can be made to cause the erasure of information in the memory comment.